1. Field of the Invention
This invention relates generally to uncooled infrared detectors and focal plane arrays, and more specifically to a method of fabricating a detector using rock salt as a removable substrate. The invention relates even more specifically to a method of fabricating a colossal magneto-resistive detector using a rock salt structure material.
2. Description of Related Art
Uncooled infrared thermal detectors have recently been developed into large-size focal plane arrays (hereinafter “FPA”). Use of a microbolometer is one successful method for infrared detection at room temperature. A microbolometer-type FPA typically employs vanadium oxide on silicon nitride with a micro-mechanically machined air bridge structure. The temperature coefficient of resistance for vanadium oxide is approximately 2%. The air bridge structure is built directly on a silicon readout integrated circuit (“ROIC”). Growth of detector materials directly on the ROIC restricts the material thin film growth temperature to less than 550° C. as a result of the thermal budget limitation associated with the ROIC.
The use of colossal magneto-resistive (“CMR”) materials for uncooled infrared detectors is described in Goyal et al., A., AMaterial Characteristics of Perovskite Manganese Oxide Thin Films for Bolometric Applications, ′ Applied Physics Letters, Vol. 71 (17) (27 Oct. 1997), pp. 2535–2537. CMR materials demonstrate an exceptionally large change in resistance with temperature as they transition from a ferromagnetic to a non-ferromagnetic phase. The transition temperature can be adjusted through appropriate selection of materials and process conditions. The results have demonstrated the feasibility of growing CMR thin films on perovskite oxide material substrates such as LaAlO3 and SrTiO3 with a resultant temperature coefficient of resistance of greater than 7%. However, the temperature for growth of the CMR material, however, must be relatively high (i.e., greater than 700° C.), which makes it difficult to grow directly on the ROIC.
CMR materials have a perovskite crystal structure with a square base. The lattice constant “a” of the square base of a CMR material is approximately 3.8 to 3.9 Å depending on the material composition. As indicated above, CMR thin films have been successfully grown on perovskite oxide substrates such as LaAlO3 and SrTiO3, and exhibit a good crystal orientation and a high temperature coefficient of resistance. These perovskite oxide substrate materials are employed because of the correspondence of their crystal structure and lattice constant to those of CMR materials. For example, SrTiO3 has a cubic crystal structure with a lattice constant of 3.905 Å, and LaAlO3 has a pseudo-cubic crystal structure with a lattice constant of 3.79 Å. These properties facilitate the growth of a CMR material on LaAlO3 and SrTiO3 with a resultant high crystal orientation and quality. The detector material can be bonded to a ROIC, then the substrate is removed. A disadvantage associated with use of these materials, however, is that both LaAlO3 and SrTiO3 are very difficult to remove by etching once the detector array has been bonded to the ROIC.
Therefore, a general need exists to provide a method of fabricating an uncooled infrared detector which both satisfies the temperature coefficient of resistance and fabrication temperature constraints, and also provides a detector of the requisite film quality. An even more specific need exists to provide a CMR transferred thin film method in which the substrate can be easily removed.